Idea for small low cost bi foiler

Discussion in 'Sailboats' started by pabloblue, Sep 28, 2012.

  1. pabloblue
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    pabloblue Junior Member

    I am new here and am looking for helpfull comments and suggestions before I start construction of a small simple bi foiler I have designed.

    My design, is to be simple and cheap home construction (under 1000 UK pounds), and relatively easy to sail (easier than Int. Moth but harder than Laser).

    Using a s/h 6-7 sq m windsurfer rig with stays on an aluminium tube superstructure with foam hulls/foats. LOA about 4m and 2.5m wide.

    The foils come from a "Pumpabike" (see YouTube) and are 2.3m x 140mm main foil and 900 x 80mm front foil.

    I am not trying to make a speed machine. It is designed for inland lakes (I am in the Lake District, UK) not on the sea.

    What is your opinion - will it fly?

    I hopefully have attached an image of the design. I would have liked to attach a SketchUp file of the 3D model but it is not a valid extension for posting. The image shows 2 waterlines, one for displacement mode and one for foiling. The front foil is shown in 2 positions, one for each waterline.

    Thanks in advance for any help and to all those who have contributed to the Foiler design and Moth on foils threads which have been so helpfull already.
     

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  2. Doug Lord
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    Doug Lord Flight Ready

    =====================
    Congratulations on giving it a go! I'd need to know what the sailing weight and foiling and non-foiling wetted surface is to make a simple assesment of "flyability". One thing is that is that the front foil has not been used succesfully in an open class like the Moth. Bethwaite did a foiling 49er with a forward foil. You have extra wetted surface if you plan on using a "normal" rudder, a forward rudder/foil , ama foils and the daggerboard. Give serious consideration to reducing surface penetrations to two(ideal) or three(maximum). Let me know your accurate weight and exact wetted surface before the boat foils and when it is foiling(both sides of all foils and immersed vertical struts) and I'll look at it further.
    As a preliminary consideration, the weight in pounds divided by sail area in sq. ft. should be less than 3 to assure foiling.
    Good Luck!
     
  3. Petros
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    Petros Senior Member

    looks like fun. keep it simple, keep it light. if you are not worried about speed why bother with the foils? Perhaps build the tri without foils and sort it out, have fun with it. And if it all works out, add the foils later. This way you are not sorting out both the hull, the sails and the foils all at once.
     
  4. pabloblue
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    pabloblue Junior Member

    Thanks for the responses. To Petros, in answer to "bother with the foils?" this is really my main aim, although I would probably would first check it will sail as a trimaran before installing the foils. I would like a fun bi foiler that is fast but I have no intention of breaking speed records. If I can have a fun bi foiling experience with a top speed of 10-15 knots i would be very happy.

    In response to Doug Lord's questions: -

    My estimate for all up boat and rig weight is <50kg, plus crew at 75kg (125 kg = 275lb)

    Total wetted area displacement mode = 3.42 sq m (36.8 sq ft.) with daggerboard
    Total wetted area displacement mode = 3.16 sq m (34 sq ft.) without daggerboard

    Total wetted area foiling mode = 1.1 sq m (11.8 sq ft.)

    The foils I intend to use can be reduced in area easily but not increased. (see photo)

    One question I have regarding reducing wetted area, do you consider I would have enough directional stability if I omit the daggerboard from the design entirely and use the main foil vertical struts only?
    The total wetted area of the main foil vertical struts is 0.2 sq m (2.15 sq ft.) reducing to 0.08 sq m (0.86 sq ft.) in foiling mode (600mm deep and 100mm chord, one each side).

    Your calculation "the weight in pounds divided by sail area in sq. ft. should be less than 3 to assure foiling" gives 275lb divided by (my 6.4 sq m rig) 69 sq ft. equals 3.98 but if I increase my rig size to 8 sq m this comes down to 3.2. Still above your figure of 3 but this seems to take no account of wind speed. I intend to have a range of rigs for different wind strenghts. I could take the rig up to 8.5 - 9 sq m to give a figure of under 3 but am concerned about it all getting a bit top heavy. Why does this calculation not include wind speed variable?

    My main reason for using a front foil is that I already have it and seems a simple low tech way of controlling ride height.

    Please note I have not drawn the front foil sensor in my 3D model although it is shown in the photo.

    Thanks again for the help.
     

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  5. Doug Lord
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    Doug Lord Flight Ready

    --------------------------------------
    W(in lb)/SA(in sq.ft) is just for rough estimates but it is interesting to note that almost every foiler fits between 2 and 3. Windspeed is irrelevant for this comparison. Bosts that fit within these parameters are: Moths, The R class(about 3.1 est upwind SA), The Rave, Osprey, the 26' Mirabaud, and most amazingly Hydroptere(with max SA) and the AC 72's with downwind SA.
    Go ahead and calculate SA/ws for your boat with and without the daggerboard foiling and not foiling in the configuration you like best. Here are some stats for other boats. Another stat that is interesting is main foil loading-the Moth is 175 lb.sq.ft.. You arrive at that by taking the total weight, multiplied by .8 and then divided by one side of the main foil. Now, on boats like the Moth the mainfoil is designed to support about 80% of the weight. I would imagine its the same with the canard. You could calculate that from the way the foils were used previously.
    One other thing: if the front foil is not used to steer and you use an aft rudder you may encounter some strange reactions from the front foil when you turn-just something to be aware of. Also turning will be slower....
    --------
     
  6. Doug Lord
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    Doug Lord Flight Ready

    It seems that you're going to have to use the smaller forward foil and I have not studied that configuration at all-just minimally in passing. Bethwaites 49'er
    used a forward "canard" but it was also the rudder. He used a surface sensing wand on the forward foil for altitude control.
     

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  7. pabloblue
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    pabloblue Junior Member

    Thanks for the assistance again. I am in the process of doing a slight re-design incorporating some ideas from here before starting constuction.

    My foil loading is 70lb/ sq, foot. Very different from the figure Doug quoted but I am unsure of the implications of this. Can you help?

    Does anyone have any idea what the required area for the daggerboard(s) for this boat. It needs to go upwind, but does not need to point very close to the wind.

    The front foil will probably pivot to follow the direction of travel to avoid working against the rudder.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ------------------------------
    That foil loading sounds excellent. I'd sail without the daggerboard and see how the vertical foil struts work.
    EDIT- I agree with everything Mr. Perry says. Your mainfoil loading is low compared with a modern Moth but that could make it easier for you to take off in lighter wind. I'd try it like it is before reducing the area. For the record Moth main foil area is about 1.1 sq.ft..
     
  9. John Perry
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    John Perry Senior Member

    Hi Pabloblue
    I was probably the first person to build and fly a 'bifoiler' and, like your proposal, the two boats I made used a canard arrangement with a front foil controling the flying height rather than the later Moth class arrangement which has flaps on a more centrally located main teefoil, these flaps controlled by a surface sensing wand. For both my boats I made the front foil as a surface piercing Vee shaped foil so that it inherently tended to run at a steady height depending on speed and loading. I see you are proposing a tee foil at the bow, presumably with some means of height control, perhaps just planing on the surface when the boat is 'flying'? If that is what you have in mind, I think it is worth a try.

    A few comments based on my experiences, although these were many years ago now.

    Your main foil looks huge - you will be amazed how much lift you can get per area of foil and you probably dont need that much lifting area unless you feel that you want to be able to 'fly' at unusually low speeds. What is the point of 'flying' at speeds that are no more than you could achieve with a much simpler and more robust conventional sailing dinghy? My guess is that a Pumpabike was designed for somethng like walking speed, but sailing bifoilers tend to go much faster than that - even when you dont really want them to. The foil system provides lots of lift and reduces craft drag well befor you reach the actual take off speed, so you will probably find you are doing 10knots before you know it and at that speed your foils would be larger than they probably need to be with consequent unnecessary drag. The foils on both my two craft were by chance about the same total lifting area as a modern Moth foiler and that is probably a good size for starters.

    You have a conventional daggerboard, presumably for use when 'taxying' in hullborne mode. I suspect you wont need that, your numerous other vertical foil surfaces should provide enough area to resist lateral sailforce. However, it would be easy to take the daggerboard up and leave it ashore, so perhaps no harm in including it, although the daggerboard case is some extra complication and weight.

    It would seem that you will steer your craft with a stern rudder which is additional to your various other foils, so perhaps extra immersed area slowing you down? Also, I have a feeling that the front foil and the struts of the main foil aft of it will together provide a fair degree of directional stabilty and you may find that the rudder has to work rather hard to overcome that when doing manouvers such as tacks and gybes. This may be particualarly the case when you are in displacement mode with all the vertical foil struts quite deeply immersed. I say this because difficulty in tacking and low speed manourvering when hull borne was one of the main failings of my craft. I did draw up a third prototype but never built it, that one would have had a bow steering canard foil and a main lifting tee foil roughly where you have drawn your main foil, that might have solved the problem.

    A windsurfer rig is the easy way to go, but rightly or wrongly I resisted that since I felt that the potential righting moment of this kind of craft would be greater than a windsurfer and hence I should have a more robust and generally stiffer rig. My first prototype did actually use a windsurfer sail, I think it was about 6.0m2 which was the largest that was commercially available in those days. However, I beefed up the mast and fitted a horrible arrangement of diamond stays - much too complicated and too many thin wires to cause injury in a crash. My second prototype had a custom made 8.00m2 sail on a custom made carbon fibre mast. This rig differed from a windsurfer rig in that it was a stiffer mast and it was designed to be used with a trapeze wire, but I never developed the skill to use it that way so I probably didnt get the full potential from it.

    One thing I thought about a lot before building my craft was how I would be able to recover it after a capsize. My first prototype had no side floats at all, so it could potentially be recovered and 'waterstarted' in the manner of a sailboard. I never developed the skill to do waterstarts so for the second prototype I fitted small trimaran floats but I made sure to keep these to a low bouyancy so that when I climbed back on board from the water the downside float would fully submerge, making the craft come upright almost automatically. Your boat looks to have fairly high bouyancy floats which will certainly make it easier to sail in displacement mode, but if it does a high speed somersault and happens to land upside down (mine was certainly capable of such aerobatics) then the floats could make it stable the wrong way up.

    I certainly dont want to sound discouraging. I well remember that when I was building my bifoilers pretty well everyone who saw what I was doing told me that I was doing it all wrong. So many people (including one highly competent and respected aerospace engineer/Americas cup designer) told me that a bifoiler could never work and that I needed to add at least a third foil for lateral stabilty! So good luck with it, and ignore everything that myself and others here tell you if you feel that you have new ideas that are at least worth a try!
     
  10. pabloblue
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    pabloblue Junior Member

    John,
    Thanks for your interesting response. Firstly let me say I had seen your 'Development of a hydrofoil sailing dinghy' web page and have stolen ideas/taken inspiration from it. I was particularly struck by you saying "For the hull I first imagined nothing more than a length of 50mm diameter aluminium tube which would make the boat a 'sinker' in sailboard parlance." I had started thinking about an Int. Moth with no hull but a aluminium tube instead. Quickly deciding that if it was possible at all to sail and water start, it was probably not within my capabilities. I then moved on to the idea of an 'easy' to sail bi foiler. Secondly I showed your web page to my brother whilst discussing the project, and he thought he had seen your foiler boat at Grafham Water SC. Have you sailed it there? I and my family spent a lot of time racing dinghies there from the 1960's untill the 1990's. Thirdly if you were also at Grafham Water, I wondered whether when you mentioned a "one highly competent and respected aerospace engineer/Americas cup designer" you were referring to my late father Dick Landon?

    Anyway on to the points you raised: -

    Main Foil size: The size I have stated is the max available to me. It would take 5 mins to remove the blue foil tips and reduce the main foil to 1270mm by 140mm chord, and easily reversible. The middle section of the foil is extruded aluminium and could also be easily cut down to reduce the area although obviously this is not reversible! The Pumpabike is rated at operating at 3 - 16 knots, although I do not know what limits the top end, I suspect it is due to muscle energy to power the bike rather than the foils. So I will start by using the full foil size in light winds and reduce after a test sail or two.

    Daggerboard: This has been cut from the design and will be added later if found to be necessary.

    Front foil and rudder: The front foil has a sensor to detect water suface and change AoA to control flying height. I am now going to add another pivot to allow it to freely rotate about a vertical axis so as to not work against the rudder. I had considered using it to steer but think the vertical strut is too small and getting a tiller from the front of the boat is a little impractical and unfamiliar to me. I realise that having 4 surface peircing foils is not ideal and will limit speed, at the moment I am most concerned with making a sailable boat not high performance.

    The rig: I am happy with using a windurfer rig mainly due to expense (I already have one) and if I need to increase sail area windsurfer rigs are very available and cheap second hand. I have no intention to trapeze, only using racks/wings for righting movement, so the loadings will be far less. I also imagine I will be sailing in less demanding conditions than windsurf rigs are designed to withstand.

    Capsizing: I intend to keep the buoyancy low - all up weight will be around 125 kg, and I will install buoyancy to float 150kg or so. I had wondered if it capsized and turned turtle i could right it by standing on the rear wing bars, sinking the transom and pulling the bow over my head, if it could not be righted in a conventional way. I have also considered attaching myself to the boat somehow to avoid it sailing away without me after a crash. Also on a practical note did you get any kind of insurance, and if so how? My sailing club requires it, but this boat not being of any class would it be obtainable?
    Sailboarders tend to insure themselves not the boards so this may be possible.

    Thanks again for your message, it has been thought provoking and not the least discouraging. I think I am close to the point where I just have to make it, test it and modify it. I will try to keep all critical points (mast step, foil mounts, etc) movable so hopefully I can come ashore and adjust if it handles like a pig!
     

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  11. John Perry
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    John Perry Senior Member

    Must have been at least ten years since I wrote that article - perhaps I better take a look at it so see if I still agree with it! The idea of a Moth without a hull has been done now, sort of anyway. You may be aware of the various versions of the 'Mirabaud' bifoiler. The first version had no hull in the usual sense of the word, just an open laticework structure. This was a much larger craft than a Moth and I suspect that they gained no advantage by not having a proper hull since the lattice work structure would probably be of comparable weight and windage to a slim light hull. I suspect they only made it that way so they could say 'Look - no hull!' Later on they modified the craft by fitting a bouyant hull underneath the open laticework structure.

    I still think it would be interesting to explore the possibilites of a small simplified bifoiler using a trapeze wire instead of Moth style sitting out racks. As I originally sugested, the hull could perhaps be just a length of tube, although I wouldnt be against having a minimalist Moth style hull. Replacing the sitting out racks by a trapeze wire would save weight and windage and the trapeze wire would directly support the mast without shrouds. Leaving off the racks should make it easy to cary down the beach and launch and easy to 'waterstart' like a sailboard. If you really want more righting moment you could of course have some small racks as well as the trapeze. I would guess that a skilled trapeze artist would be able to move their body weight around more fluently than is possible sitting on racks. I suspect that the ability to continuously and rapidly trim the craft with body weight at least in part accounts for the sucess of both windsurfers and kiteboards in speed sailing.


    Yes, I did keep my second bifoiler at Grafham water for a year or so, basically because at the time I had nowhere to store it at home. I also sailed it there about three times, including one 'flight' most of the way accross the lake - conditions must have been just right that day. Then I reduced the height of the forward foil and after that I could never even get it to fully take off. Just shows, to get anywhere with this kind of thing you probably need to be prepared to build/test/rebuild/test/rebuild/test over and over again if need be.

    No, as far as I am aware I never met your father, it was someone else I had in mind. I shall not say who because that kind of response to new ideas is very normal human behaviour - I tend to respond that way myself - if I see a new idea I find myself looking for snags or countering it with some different idea of my own, anything rather than agreeing that it might actually work! Of course, people who negatively critisize new ideas are probably right more often than they are wrong.

    Re insurance, I did insure my second bifoiler, third party only. I recall that I arranged the insurance over the phone and described the boat as an 11 foot sailing boat, mostly wooden construction. The man asked me what was the name of the boat and I told him I hadnt bothered to give it a name. "That's OK" he said, "I will put it down as T.B.A." He told me what the premium would be then I asked "don't you need to know anything more about the boat?" and he replied "No, if you only want third party that's all I need".

    Presumably you need to make the front foil assembly 'castor' - from your marked up picture of the foil assembly it looks more like it would just swing to full lock, but I expect I am missunderstanding the picture. I can see the logic behind all your other comments, I understand that you are initially designing for fairly low speed operation, then if all goes well you might look to increase performance later - that does make sense.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    All-here are some pictures of Mirabaud-the largest monofoiler so far- and the largest bi-foiler until TNZ's and Oracles 72' cats became the first multihulls to foil on just two foils. Mirabaud started as a "Foiling 18" until Thomas Jundt got funding from the Mirabaud Company. One of his experiments was to tow the hull frame on foils until it was flying and then cast off!

    Pictures: L-R, 1) the Frame, 2)-Rendering of Mirabaud with solid wing, 3) Mirabaud flying, 4) The Wing under sail and flying:

    click-
     

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  13. pabloblue
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    pabloblue Junior Member

    Thanks again for the responses. 'Mirabaud' looks very interesting. My concept is close to a minature version of this, but with trimaran stability in displacement mode.

    Could anyone explain why Mirabaud's daggerboard is so far foward. Is it positioned at the ideal place for the lifting foil rather than the daggerboard foil?
    I would have imagined positioning it there would have made the boat have excessive weather helm, particularly at low speed.

    My understanding is, in a convertional boat, the centreboard is postioned below the centre of effort of the sail, which is approx one third back from the luff. As speed increases the area required is smaller and moves toward the stern. A sailboards small fin at the rear an extreme example of this.

    When I scrapped the daggerboard in my design, and was proposing to use only the foil struts as 'daggerboards' I was concerned the boat would want to bear away from the wind but hopefully not too excessively ( I increased mast rake to conpensate a little).

    Am I misunderstanding the theory? Why doesn't Mirabaud not want to head up into wind all the time and will mine want to bear away excessively?

    Is maintaing the 80/20 percent loading on the *lifting* foils a far higher priority than postion of the *directional* foils which are fairly uncritcal?

    Sorry for all the questions, but the pictures of Mirabaud have made me question my understanding of the theory, if anyone could help explain I would appreciate it.
     
  14. Doug Lord
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    Doug Lord Flight Ready

    ====================
    Pablo, here is an article about Bill Roberts "shared lift" for the Arc 21 catamaran. http://www.aquarius-sail.com/catamarans/arc21/index.htm On that boat the daggerboard is forward of the front beam. The
    reason that works is that he makes the daggerboard a little smaller and the rudder is a little bigger. Mirabaud does the same thing-note that the vertical foils are about the same size, instead of the daggerboard being much larger as on a conventional boat.
    For a conventional bi-foiler the most critical things in positioning the daggerboard are:
    1) that the main foil supports 80% of the boats weight and,
    2) that the distance between the main foil and rudder foil is as great as possible for better pitch response.
    I'm not familiar enough with the canard configuration to speak much about it but I imagine the same princibles would apply to some degree-John could help with that.

    Picture-sketch of Moth. Note that the daggerboard is a bit further forward than it would be on a conventional dinghy, particularly if you consider the foils with the boat flying. Also note that the Moth uses a rudder gantry to extend the rudder and its T-foil as far aft as legal under the rules:
     

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  15. John Perry
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    John Perry Senior Member

    There are 'rules of thumb' for determining keel/daggerboard/centre board placement for conventional monohull sailing boats, but these are only guidelines and in any case it is probably unwise to apply such rules to unconventional designs such as we are considering here.

    Bifoiler sailing hydrofoils differ from most other sailing boats in the way that they heel when sailing to windward, and nearly always they are sailing to windward, at least relative to their apparent wind. Ballasted keelboats usually sail best to windward heeled to leeward by up to around 30 degrees. Unballasted monohulls are best sailed to windward at rather smaller angles of heel. Conventional multihulls heel little, but they do heel to leeward. Bifoilers on the other hand seem to be best sailed heeling towards the wind, not away from it. The ones I made actually became rapidly unstable if they were allowed to heel to leeward.

    The heel of a sailing boat will certainly affect the optimum placement of the underwater appendages. If you have a small monohull that has sufficient directional stability that it will sail itself with the helm locked, then you can steer it quite precisely by just adjusting the angle of heel, say by moving your weight accross the boat. As you increase the heel to leeward the boat turns into the wind and as you reduce the leeward heel, or even heel to windward, it turns away from the wind. It follows that a boat that normally sails heeled to windward would tend to turn away from the wind if it had a conventional placement of the underwater appendages. The relatively far forward position of the daggerboard as seen in Doug's picture of Mirabaud will counteract this. Also, considering Doug's side view diagram of a bifoiler Moth, when the boat is foil borne much of the lateral hydrodynamic force that oposes the lateral force from the sails is actually generated by the tilt (in the roll direction) of the main lifting foil, rather than by the foil struts. That main lifting foil is positioned relatively further forward than would be the daggerboard on a non-foiling racing dinghy.

    Actually, once a typical bifoiler is foilborne, I think that its directional control is likely to be fairly insensitive to the longitudinal positions of the under water appendages. Basically you have two separate hydrofoil units, spaced quite well apart, and no other immersed structures to resist directional changes. It probably dosen't take much steering angle on the steered hydrofoil unit relative to the fixed one to correct for a bit of a mismatch between the lines of action of the lateral sail forces and the lateral hydrodynamic forces. But to get to the situation where you are foil borne you first have to sail in displacement mode when side forces are mainly generated by foil struts rather than by the tilt of the foil lifting surfaces, and the lateral immersed profile of these struts may differ considerably from their profile when foilborne.

    My own bifoilers had too much weather helm when sailing in displacement mode. I think this was basically due to the relatively deep immersion of the forward foil assemblies when in displacement mode. The problem was exacerbated by deep immersion of the aft part of the displacement hull, so that the stern foil strut, which was also the rudder, had to push the stern of the hull sideways in the water to make a turn. I could move my body weight aft in order to pitch the bow up and reduce the immersion of the forward foil, but this only caused the stern of the displacement hull to sink deeper and cause more resistance to rudder action. Once the craft got moving at something around walking speed, still in displacement mode of course, the problem reduced a lot because there was then enough flow over the steering surfaces to give control authority, also lift from the foils soon started to reduce the immersion of the displacement hull. Once the boat was fully foilborne and heeling to windward, the main lifting foil aft would generate much of the hydrodynamic lateral force, probably eliminating any weather helm. However, tacking in displacment mode was always problematic (I never got to the stage of considering tacking while foilborne) and starting the boat moving from standstill, say after righting it from a capsize, could be infuriatingly difficult. It was to reduce the immersion of the forward foil assembly when in displacement mode that I reduced the depth of this assembly (i.e. the draft). Unfortunately this meant that the forward foil assembly could no longer lift the bow of the craft quite high enough at the point of take off to give enough angle of attack on the main lifting foil. There were certainly modifications I could have made to improve matters, but at that point I returned to my original interest in sailing which was relatively long distance cruising in small open boats, these being relatively conventional monohul boats. For example, this summer we made a relaxed and enjoyable open boat cruise from Lechlade, near the source of the river Thames in the UK, through to the Waddensea north of Holland, visiting numerous interesting places on the way. I am not sure I will ever go back to floundering around in chilly water in a wetsuit trying to find out how to make a bifoiler work properly!
     
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